Effects of defoliation and habitat type on Medusahead demography

Background/Question/Methods

The winter annual grass Taeniatherum caput-medusae (L.) Nevski, commonly known as medusahead, has affected millions of hectares of rangeland across 17 U.S. states, significantly decreasing livestock forage production, reducing biodiversity, and promoting increased wildfire frequencies. Methods to control medusahead, particularly targeted defoliation (clipping or grazing) have been tested. However, results from these efforts are highly variable, and have not been successful in the long-term. Contributing to the limited success of controlling medusahead is an absence of fundamental demographic knowledge about this noxious weed, which is essential for predicting the behavior of future invasions. By gaining a better understanding of the life cycle of medusahead, we can assess the susceptibility of life stages to different abiotic factors, and create models to assist with combating future invasions.

We examined the demography of medusahead across its life cycle within oak woodland and open grassland habitat types at different seeding rates (0, 10, 100, 10,000, and 50,000 seeds per m²), both with and without targeted defoliation, in a randomized design. Survival, growth, and fecundity data was collected to construct periodic matrix models over 1 year. Perturbation analysis was used to determine the sensitivity of each stage to experimental treatments.

Results/Conclusions

We found overall population growth rate of medusahead to be lower in oak woodland habitats compared to open grasslands, as a result of reduced germination and seedling establishment. We also found that defoliation served to decrease overall medusahead density in both habitats. However, this reduction in density led to an increase in flower production by individuals that escaped the defoliation treatment (due to a late phenology). We expect that this increased flower production will be a positive feedback on population growth for the following year.

This work highlights the importance of integrating demographic considerations into invasive weed management efforts. Specifically, our work suggests that effective eradication methods for medusahead require a series of defoliation treatments in order to target both early and late phenology individuals.